The invention relates to a blend of at least two thermoplastic components differing in their crystallinities. The blend has a heterogeneous morphology with distinct phase separated zones of different composition and crystallinity present. The resulting blend with the defined morphology shows dramatic improvements in mechanical deformation recoverability when compared to its individual unblended components. The invention also relates to improvements in the flexibility of the blend. Changes in the relative amounts or the crystallinities of the blend components or the morphology of the blend affect the recoverability and the flexibility of the blend.
The inventive blends designed for recoverability contains a dispersed phase of a greater crystallinity and a continuous phase of lesser crystallinity. The sizes of the individual domains of the dispersed phase are very small with the smallest length dimension for the dispersed phase being less than 5 xcexcm. This phase size of the dispersed phase is maintained during processing even without crosslinking. The inventive blends designed for flexibility have a slightly wider range in morphology as the components of greater and lesser crystallinity can also be co-continuous. The components of the blend in both cases are also compatible to the extent that no compatibilizer needs to be added to attain and retain this fine morphology. Furthermore, this invention describes improving the mechanical deformation recoverability of the aforementioned blends by aging the blends and mechanically orienting the articles formed from these blends.
One of the components is a polymer comprising predominately stereospecific polypropylene, preferably isotactic polypropylene. This is the component with greater crystallinity. A second component is a copolymer of propylene and a C2, C3-C20 xcex1-olefin, preferably ethylene. This is the component with lesser crystallinity. In the copolymer the propylene is polymerized substantially stereospecifically. The copolymer has a substantially uniform composition distribution preferably as a result of polymerization with a metallocene catalyst. Most preferably, said second component is an ethylene propylene copolymer, e.g. ethylene propylene semicrystalline elastomer.
There is a need in the art for polymeric blends having a stereospecific polypropylene component with good tensile strength while still providing suitable mechanical recoverability (elastic recovery) and flexibility (low flexural modulus). This invention is aimed at improving the aforementioned properties of blends having a stereoregular polypropylene component, especially isotactic polypropylene. This is achieved by blending the stereoregular polypropylene component with a copolymer of propylene and a C2, C3-C20 xcex1-olefin. This copolymer is less crystalline than the isotactic polypropylene. In the copolymer the propylene is polymerized substantially stereospecifically. Most preferably, the copolymer is an ethylene propylene copolymer, e.g., ethylene propylene thermoplastic elastomer. The copolymer has a substantially uniform composition distribution preferably as a result of polymerization with a metallocene catalyst Composition distribution is a property of copolymers indicating a statistically significant intermolecular or intramolecular difference in the composition of the polymer. Methods for measuring compositional distribution are described later.
Blends of isotactic polypropylene and ethylene propylene rubber are well known in the prior art. However, the traditional Ziegler-Natta catalysts cannot make ethylene propylene thermoplastic elastomers which simultaneously are uniform in compositional distribution, have substantially stereospecific propylene residues and have less than 35 wt. % ethylene.
U.S. Pat. No. 3,882,197 to Fritz et al. describes blends of stereoregular propylene/alpha-olefin copolymers, stereoregular propylene, and ethylene copolymer rubbers. In U.S. Pat. No. 3,888,949 Chi-Kai Shih, assigned to E I Du Pont, shows the synthesis of blend compositions containing isotactic polypropylene and copolymers of propylene and an alpha-olefin, containing between 6-20 carbon atoms, which have improved elongation and tensile strength over either the copolymer or isotactic polypropylene. Copolymers of propylene and alpha-olefin are described wherein the alpha-olefin is hexene, octene or dodecene. However, the copolymer is made with a heterogeneous titanium catalyst resulting in copolymers with non-uniform composition distribution and a broad molecular weight distribution. Non-uniform intramolecular compositional distribution is evident in U.S. Pat. No. 3,888,949 by the use of the term xe2x80x9cblockxe2x80x9d in the description of the polymer where the copolymer is described as having xe2x80x9csequences of different alpha-olefin content.xe2x80x9d Within the context of the invention described above the term sequences describes a number of olefin monomer residues linked together by chemical formed during polymerization.
In U.S. Pat. No. 4,461,872, A.C.L. Su improved on the properties of the blends described in U.S. Pat. No. 3,888,949 by using another heterogeneous catalyst system which is expected to form copolymers which have statistically significant intermolecular and intramolecular compositional differences.
In two successive publications in the journal of Macromolecules, 1989, V22, pages 3851-3866, J. W. Collette of E. I. Du Pont has described blends of isotactic polypropylene and partially atactic polypropylene which have desirable tensile elongation properties. However, the partially atactic propylene has a broad molecular weight distribution as shown in FIG. 8 of the first publication. The partially atactic polypropylene is also composed of several fractions, which differ in the level of tacticity of the propylene units as shown by the differences in the solubility in different solvents. This is shown by the corresponding physical decomposition of the blend which is separated by extraction with different solvents to yield individual components of uniform solubility characteristics as shown in Table IV of the above publications.
More recently several authors have shown the formation of more refined structures of partially atactic, partially isotactic polypropylene which have elastomeric properties. It is believed that in these components each molecule consists of portions which are isotactic and therefore crystallizable while the other portions of the same polypropylene molecule are atactic and therefore amorphous. Examples of these propylene homopolymers containing different levels of isotacticity in different portions of the molecule are described by R. Waymouth in U.S. Pat. No. 5,594,080, in the article in the Journal American Chemical Society (1995), Vol. 117, page 11586, and in the article in the Journal American Chemical Society (1997), Vol. 119, page 3635, J. Chien in the journal article in the Journal of the American Chemical Society (1991), Vol. 113, pages 8569-8570; and S. Collins in the journal article in Macromolecules (1995) Vol. 28, pages 3771-3778. These articles describe a specific polymer, but do not describe the blends with a more crystalline polymer such as isotactic polypropylene.
In U.S. Pat. Nos. 3,853,969 and 3,378,606, E. G. Kontos discloses the formation of in situ blends of isotactic polypropylene and xe2x80x9cstereo blockxe2x80x9d copolymers of propylene and another olefin of 2 to 12 carbon atoms, including ethylene and hexene. The copolymers of this invention are necessarily heterogeneous in intermolecular and intramolecular composition distribution. This is demonstrated by the synthesis procedures of these copolymers which involve sequential injection of monomer mixtures of different compositions to synthesize polymeric portions of analogously different compositions. In addition, FIG. 1 of both patents shows that the xe2x80x9cstereo blockxe2x80x9d character, which is intra or intermolecular compositional differences in the context of the description of the present invention, is essential to the benefit of the tensile and elongation properties of the blend. Moreover, all of these compositions either do not meet all of the desired properties for various applications, and/or involve costly and burdensome process steps to achieve the desired results.
Similar results are anticipated by R. Holzer and K. Mehnert in U.S. Pat. No. 3,262,992 assigned to Hercules wherein the authors disclose that the addition of a estereoblock copolymer of ethylene and propylene to isotactic polypropylene leads to improved mechanical properties of the blend compared to isotactic polypropylene alone. However, these benefits are described only for the stereoblock copolymers of ethylene and propylene. These copolymers were synthesized by changing the monomer concentrations in the reactor with time. This is shown in examples 1 and 2. The stereoblock character of the polymer is graphically shown in the molecular description (column 2, line 65) and contrasted with the undesirable random copolymer (column 2, line 60). The presence of stereoblock character in these polymers is shown by the high melting point of these polymers and the poor solubility in hydrocarbons at ambient temperature.
There is a need for a polyolefin blend composition which is thermally stable, heat resistant, light resistant and generally suitable for thermoplastic elastomer (TPE) applications which has advantageous processing characteristics. We have found that by blending a crystalline propylene polymer, hereinafter referred to as the xe2x80x9cfirst polymer component, (FPC)xe2x80x9d and a crystallizable propylene alpha olefin copolymer polymer, hereinafter referred to as the xe2x80x9csecond polymer component (SPC)xe2x80x9d, advantageous processing characteristics result while still providing a composition having decreased flexural modulus and increased tensile strength, elongation, recovery and overall toughness. It is possible to have the addition of a third polymeric component which is another crystallizable propylene alpha olefin copolymer indicated as SPC2 in the text below which has crystallinity intermediate between the FPC and the, SPC. The SPC2 also has a narrow composition distribution and is made with a metallocene catalyst. The addition of SPC2 leads to a finer morphology and improvements in some of the properties of the blend of FPC and SPC.
The term xe2x80x9ccrystalline,xe2x80x9d as used herein for FPC, characterizes those polymers which possess high degrees of inter- and intra-molecular order, and which melt higher than 110xc2x0 C. and preferably higher than 115xc2x0 C. and more preferably higher than 130xc2x0 C. and preferably have a heat of fusion of at least 75 J/g, as determined by DSC analysis. And, the term xe2x80x9ccrystallizable,xe2x80x9d as used herein for SPC describes polymers which are mainly amorphous in the undeformed state, but can crystalize upon stretching or annealing. Crystallization may also be initiated by the presence of a crystalline polymer such as the FPC. These polymers have a melting point of less than 105xc2x0 C. or preferably less than 100xc2x0 C. and preferably have a heat of fusion of less than 75 J/g as determined by DSC. SPC2 describes those polymers that are substantially crystalline in the undeformed state. Further crystallization may also occur in the presence of the crystalline polymer such as FPC. These polymers have a melting point of less than 115xc2x0 C. or preferably less than 100xc2x0 C. and preferably have a heat of fusion of less than 75 J/g as determined by DSC.
The present invention is directed to blends with heterophase morphology formed by blending a FPC which is a predominately crystalline stereoregular polypropylene with a SPC which is a crystallizable copolymer of a C2, C4-C20 xcex1-olefin (preferably ethylene) and propylene . Optional components of the blend are SPC2, a crystallizable copolymer of a C2, C4-C20 xcex1-olefin (preferably ethylene), and process oil. Other optional components are fillers, colorants, antioxidants, nucleators and flow improvers.
The FPC melts higher than 110xc2x0 C. and preferably higher than 115xc2x0 C. and more preferably higher than 130xc2x0 C. and preferably has a heat of fusion of at least 75 J/g, as determined by DSC analysis. The crystalline polypropylene can be either homopolymer or copolymers with other alpha olefins. The FPC may also be comprised of commonly available isotactic polypropylene compositions referred to as impact copolymer or reactor copolymer. However these variations in the identity of the FPC are acceptable in the blend only to the extent that the FPC is within the limitations of the crystallinity and melting point indicated above. The FPC may also contain additives such as flow improvers, nucleators and antioxidants which are normally added to isotactic polypropylene to improve or retain properties. All of these polymers are referred to as the FPC.
The SPC and the SPC2, if used, have stereoregular propylene sequences long enough to crystallize. The SPC has a melting point of less than 105xc2x0 C. or preferably less than 100xc2x0 C. and preferably has a heat of fusion of less than 75 J/g. The SPC2 has a melting point of less than 115xc2x0 C. or preferably less than 100xc2x0 C. and preferably has a heat of fusion of less than 75 J/g. These stereoregular propylene sequences of SPC and SPC2 should substantially match the stereoregularity of the propylene in the first polymer. For example, if the FPC is predominately isotactic polypropylene, then the SPC, and SPC2 if used, is copolymer having isotactic propylene sequences. If the FPC is predominately syndiotactic polypropylene, then the SPC, and the SPC2 if used, is a copolymer having syndiotactic sequences. Therefore, SPC and SPC2 have similar, preferably substantially identical, tacticity to the FPC. It is believed that this matching of stereoregularity increases the compatibility of the components and results in improved adhesion at the interface of the domains of the polymers of different crystallinities in the polymer blend composition. Furthermore, good compatibility is only achieved in a narrow range of copolymer composition for the SPC. Narrow intermolecular and intramolecular compositional distribution in the copolymer is preferred. The aforementioned characteristics of the SPC, and SPC2 if used, are preferably achieved by polymerization with a chiral metallocene catalyst.
One preferable embodiment is blending isotactic polypropylene (FPC) with ethylene propylene copolymers (SPC) having about 4 wt. % to about 35 wt. % ethylene (to ensure high compatibility with the FPC). Both the FPC and the SPC have isotactic propylene sequences long enough to crystallize. Resulting blends of isotactic polypropylene with ethylene propylene copolymers according to the invention have improved properties as compared to isotactic polypropylene blends with prior art ethylene propylene rubbers.
A preferred blend comprises 1% to 95% by weight of FPC and a SPC with greater than 65% by weight propylene and preferably greater than 80% by weight propylene.
According to another embodiment, a thermoplastic polymer blend composition of the invention comprises a FPC and a SPC with added process oil. The FPC comprises isotactic polypropylene, a reactor copolymer or an impact copolymer as described above and is present in an amount of about 1% to about 95% by weight and more preferably 2% to 70% by weight of the total weight of the blend. The balance of the polymer composition consists of a mixture of the process oil and the SPC and SPC2 if used
The SPC is a random copolymer of ethylene and propylene having a melting point by DSC of 0xc2x0 C. to 105xc2x0 C., preferably in the range 20xc2x0 C. to 90xc2x0 C., more preferably in the range of 25xc2x0 C. to 700xc2x0 C. and an average propylene content by weight of at least 65% and more preferably at least 80%. This melting point is due to crystallizable propylene sequences, preferably of isotactic polypropylene. The SPC is made with a polymerization catalyst which forms essentially or substantially isotactic polypropylene, when all or substantially all propylene sequences in the FPC are isotactic. The SPC is statistically random in the distribution of the ethylene and propylene residues along the chain. Quantitative evaluation of the randomness of the distribution of the ethylene and propylene sequences may be obtained by consideration of the experimentally determined reactivity ratios of the second polymer component or by 13 C NMR. This is according to the procedures described in the journal article by H. Kakugo, Y Naito, K. Mizunama and T. Miyatake in Macromolecules (1982), pages 1150-1152. The SPC is made with a single sited metallocene catalyst which allows only a single statistical mode of addition of ethylene and propylene by polymerization in a well mixed, continuous feed stirred tank reactor which provides a uniform polymerization environment for growth of all of the polymer chains of the SPC.
The ratio of the FPC to the SPC of the blend composition of the present invention may vary in the range of 1:99 to 95:5 by weight and more preferably in the range 2:98 to 70:30 by weight.
According to another embodiment of the present invention, the second polymer component may contain small quantities of a non-conjugated diene to aid in the vulcanization and other chemical modification of the blend of the first polymer component and the second polymer component. The amount of diene is preferably less than 10 wt. % and preferably less than 5 wt. %. The diene may be selected from the group consisting of those which are used for the vulcanization of ethylene propylene rubbers and are preferably ethylidene norbornene, vinyl norbornene and dicyclopentadiene.
The SPC2, if used, has the same characteristics as the SPC described above. The SPC2 has a crystallinity and composition intermediate between the FPC and the SPC. In the preferred case where the SPC2 is a copolymer of ethylene and propylene while the FPC is homopolymer of propylene. The SPC2 has the same type of crystallinity of propylene as in the FPC and SPC and an ethylene content in between FPC and SPC. The addition of SPC2 to the blend leads to a better dispersion of the phases in the blend compared to blends of the similar composition which do not have any SPC2. The relative amounts of SPC and SPC2 can vary between 95:5 to 10:90 in the blend. The ratio of the FPC to the sum of SPC and SPC2 may vary in the range of 1:99 to 95:5 by weight and more preferably in the range 2:98 to 70:30 by weight
According to still a further embodiment, the invention is directed to a process for preparing thermoplastic polymer blend compositions. The process comprises: (a) polymerizing propylene or a mixture of propylene and one or more monomers selected from C2 or C3-C20 xcex1-olefins in the presence of a polymerization catalyst wherein a substantially isotactic propylene polymer containing at least about 90% by weight polymerized propylene is obtained; (b) polymerizing a mixture of ethylene and propylene in the presence of a chiral metallocene catalyst, wherein a copolymer of ethylene and propylene is obtained comprising up to about 35% by weight ethylene and preferably up to 20% by weight ethylene and containing isotactically crystallizable propylene sequences; and (c) blending the propylene polymer of step (a) with the copolymer of step (b) to form a blend.
The invention is directed to the formation of a blend of the components FPC, SPC and SPC2 which has a heterogeneous phase morphology consisting of domains of different crystallinities. Blends directed to improvement in the elastic recovery have a continuous phase of lower crystallinity and a dispersed phase of the higher crystallinity. The domains of the dispersed phase are small with an average minimum axis less than 5 xcexcm. The larger axis of the dispersed phase can be as large as 100 xcexc. The dispersed phase consists of a crystalline mixture of FPC with some amount of SPC2 and SPC due to thermodynamic mixing of polymers. The continuous phase consists of the balance of the polymers not included in the dispersed phase. Blends directed to low flexural modulus may have in addition, a heterogeneous phase morphology with continuous phases of lower and greater crystallinity.
Commonly available propylene reactor copolymers consisting of a single phase blend of isotactic polypropylene and copolymers of propylene and ethylene are not included within the scope of the invention since they are a single phase with no prominent dispersed or continuous phases. Polypropylene blends made by a combination of a FPC and a SPC of the present invention that give a heterophase morphology in which the crystalline polymer is the continuous phase in are excluded from the invention.
The benefits of the invention are included improvement in the elastic recovery and the flexural modulus of the blend. These improvements are most apparent as a function of the 500% tensile modulus of the blend. Historically, the examples of the prior art have been able to duplicate the improvements in the blend but only for compositions with a very low 500% tensile modulus